Process for removing relatively coarse-grained solids from a stationary fluidized bed

ABSTRACT

According to the invention, a fluidizing gas is introduced upwards into the fluidized bed ( 3 ) through a valve grid ( 2 ), said stationary fluidized bed ( 3 ) containing solids with different grain sizes. A supply pipe ( 10 ) is disposed in the area of the fluidized bed, the mouth of said pipe being located above the valve grid ( 2 ) and its outlet ( 12 ) leading outwards from the fluidized bed ( 3 ). Part of the solids located above the valve grid ( 2 ) is blown into the supply pipe by a gas jet ( 13 ) which is directed upwards and fed separately from the fluidizing gas and pneumatically evacuated from the fluidized bed through the supply pipe ( 10 ). Normally, the flow speed of the gas in the supply pipe is at least two times higher than the speed of the fluidizing gas in the fluidized bed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/EP 99/04526 filed Jul. 1,1999 and based, in turn, upon German national application 198 30 697.0filed Jul. 8, 1998 under the International Convention.

FIELD OF THE INVENTION

This invention relates to a process for removing relativelycoarse-grained solids from a stationary fluidized bed, which containssolids of different grain sizes, wherein fluidizing gas is introducedinto the fluidized bed from the bottom, distributed through a jet bank.

BACKGROUND OF THE INVENTION

Stationary fluidized beds of this kind are generally known. They arecharacterized by a more or less precisely defined bed surface, whichmoves for instance like a boiling liquid. During an extended operationof the fluidized bed solids of a relatively large grain size accumulatein the lower portion of the bed above the jet bank, as they are notfluidized or are hardly fluidized. This accumulation of coarse grain canbe disturbing in many cases, as it can for instance lead to differentdwell times of the solids in the fluidized bed and also to anoverheating and sintering in the vicinity of the jet bank. Thewithdrawal of this coarse grain under the influence of gravity downwardsthrough the jet bank or laterally through the container wall requires aconsiderable effort in terms of equipment and control technology.

OBJECT OF THE INVENTION

It is the object of the invention to be able to at least partly andeasily remove the relatively coarse-grained solids accumulating in thelower portion of a stationary fluidized bed.

SUMMARY OF THE INVENTION

In accordance with the invention this is achieved in the above-mentionedprocess in that by means of at least one upwardly directed gas jetsupplied separate from the fluidizing gas part of the solids disposedabove the jet bank are blown into a conveying tube, and these solids arepneumatically removed from the fluidized bed through the conveying tube.Since the relatively coarse-grained solids accumulate directly above thejet bank, the same are, as far as they are disposed in direct vicinityof the inlet opening of the conveying tube, blown out of the fluidizedbed through the conveying tube together with the fine-grained solidspresent near the inlet opening. The solids discharged in this way car,be separated into a coarse grain fraction and a fine grain fraction byscreening them outside the fluidized bed, where for instance the finegrain fraction is directly recirculated to the fluidized bed and thecoarse grain fraction is first of all comminuted or subjected to aspecial treatment in a second container.

The solids in the fluidized bed may have any temperature, the spectrumof grain sizes may include grain sizes in the range from 0.1 to 10 mm,but there are no absolute size limits for this method.

The gas velocities of the fluidizing gas usually lie in the range from0.1 to 1 m/sec, where these figures are measured as empty-tubevelocities, here and later on. One or several gas jets are upwardlydirected from the bottom against the inlet opening of the conveyingtube, they entrain the solids and guide them through the conveying tube,through which they leave the fluidized bed. The gas velocities in theconveying tube usually are at least twice as high as the velocity of thefluidizing gas in the fluidized bed. Usually, the gas velocities in theconveying tube are twice to five times as high as in the fluidized bed.

It may be sufficient to install in the fluidized bed a single conveyingtube with at least one associated gas jet coming from the bottom, butthere may very well also be provided a plurality of conveying tubes atdifferent points in the fluidized bed, to each of which at least one gasjet is associated.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the method will be explained with reference to thedrawing, wherein:

FIG. 1 shows a vertical section along line II—II of FIG. 2 through afluidized-bed reactor in a schematic representation;

FIG. 2 shows a horizontal section along line I—I through the reactorillustrated in FIG. 1; and

FIG. 3 shows a vertical section through a fluidized-bed reactor with twoconveying tubes.

SPECIFIC DESCRIPTION

The reactor 1 illustrated in FIGS. 1 and 2 has a stationary fluidizedbed 3 above a jet bank 2. Solids of different grain sizes are chargedinto the reactor through line 4, and fluidizing gas is supplied throughline 5. The fluidizing gas first of all enters a distribution chamber 6,before it flows through the jet bank 2 into the fluidized bed 3. Thefluidized tied has a solids surface 8, through which the gas escapes inupward direction, where a certain amount of the finegrained solids isentrained. The gas leaves the reactor through the outlet 9, and issupplied for instance to a dedusting not represented here.

At the wall of the reactor 1, see FIGS. 1 and 2, a conveying tube 10 isprovided, which has an inlet opening 11 and an outlet 12. The inletopening 11 lies above the jet bank 2 by the vertical distance (a), where(a) usually is 10 to 500 mm. Below the conveying tube 10 and alsodirectly below the jet bank 2 a vertical gas line 13 is provided,through which in a manner not represented here a gas jet is blown fromthe bottom upwards through the inlet opening 11 into the conveying tube10. The upper end of the line 13 is disposed approximately at the levelof the jet bank 2, so that solids from the fluidized bed 3 constantlyflow into the range of influence of the gas jet, which carries the sameinto the conveying tube 10 and to the outlet 12. It may be expedient tosupply the solids coming through the outlet 12, which are relativelycoarse-grained, to a comminution or to a not represented secondfluidized bed. The fine grain component in the solids may first bescreened out.

The number of the conveying tubes installed in the fluidized bed of areactor may be chosen as desired. In FIG. 3, two conveying tubes 10 aand 10 b are represented, to which gas lines 13 a and 13 b,respectively, are associated. The conveying tube 10 a is slightlyinclined towards the top, and it protrudes beyond the surface 8 of thefluidized bed. On the other hand, the upper bend 15 of the conveyingtube 10 b lies in the fluidized bed 3 slightly below its surface 8. Theinlet openings 11 a and 11 b of the two conveying tubes shown in FIG. 3are slightly flared towards the bottom, but this is not absolutelynecessary. The remaining reference numerals of FIG. 3 have the meaningexplained already in conjunction with FIGS. 1 and 2.

EXAMPLE

Into a continuously operated fluidized bed reactor, which is designed inaccordance with FIGS. 1 and 2, 1125 kg/h quartz sand are charged. 90wt-% of the sand have a grain size of to 2.2 mm. The jet bank 2 has asurface area of 4.2 m2, the height of the bed is 0.6 m. For fluidizingpurposes air is used, whose empty-tube velocity in the vicinity of thebed is 0.2 m/s. In the vicinity of the orifice 11, the air supplied tothe conveying tube 10 through line 13 has an empty-tube velocity of 0.6m/s. The conveying tube has a cross-sectional area of 166 cm², itsdistance (a) from the jet bank is 100 mm.

During an operation of 4 hours, the sand blown out through the conveyingtube is collected in a container. The total amount of this sand is 270kg, of which 180 kg (=66.7 wt-%) have a grain size of 0.5-2.2 mm, sothat preferably coarse-grained solids have been discharged.

What is claimed is:
 1. A process for removing relatively coarse grainedsolids from a stationary fluidized bed, comprising the steps of: (a)fluidizing a stationary fluidized bed above a jet bank by introducing afluidizing gas into said bed at a bottom thereof through jets of saidjet bank; (b) providing an upright conveying tube in said fluidized bedabove said jet bank, said conveying tube having an inlet opening closeto but above said jet bank at a distance of 10 to 500 mm therefrom; (c)directing a jet of a conveying gas upwardly into said conveying tubeindependently of the fluidizing gas and gas discharged by the jets ofsaid jet bank, from a vertical gas line spaced below said inlet opening,said vertical gas line having an upper end from which said conveying gasis discharged located at approximately a level of said jet bank, wherebyrelatively coarse grained solids from said bed are entrained upwardly insaid conveying tube by said conveying gas; (d) passing said relativelycoarse grained solids entrained upwardly in said conveying tube by saidconveying gas to and through a bend at an upper end of said conveyingtube and then downwardly out of said fluidized bed; and (e) adjusting agas velocity in said conveying tube to be at least twice as high as afluidizing gas velocity in said fluidized bed.
 2. The process defined inclaim 1, further comprising the step of: (f) introducing the relativelycoarse grained solids entrained upwardly in said conveying tube by saidconveying gas to and through said bend at said upper end of saidconveying tube and then downwardly out of said fluidized bed into asecond fluidized bed.